1. Field of the Invention
The present invention relates to an electromagnetic flowmeter to generate a magnetic field in a direction perpendicular to measured fluid flowing through a measurement tube, and detect electromotive force generated according to a flow rate of the fluid intersecting the magnetic field through one pair of electrodes diametrically opposed on the measurement tube so as to measure the flow rate.
2. Description of the Prior Art
FIG. 4 is a sectional view showing a conventional electromagnetic flowmeter 50, and FIG. 5 is a perspective view showing a state in which a comparatively wide caliber electromagnetic flowmeter 150 is mounted between ducts 11-1 and 11-2. In FIG. 4, reference numeral 51 is a non-magnetic measurement tube made of stainless alloy, through which measured fluid serving as conductive liquid can pass, 52a and 52b are coils serving as magnetic field generating means opposed on a diameter of the measurement tube 51 such that a magnetic field can be generated in the measurement tube 51 to have a direction perpendicular to a direction in which the measured fluid flows, 53 is a pair of electrodes opposed on the diameter or the measurement tube 51, and 54 is a steel case serving as a detector obtained by integrally forming an opening 54-1 for accommodating the measurement tube 51, spaces 54-2a and 54-2b for accommodating the coils 52a and 52b, and so forth. Further, reference numerals 55a and 55b are brackets forming a closed magnetic circuit serving as a feedback circuit with magnetic flux .PHI. between the coils 52a, 52b and the case 54.
A lining material 56 such as fluororesin is applied to an inner surface and an end surface of the measurement tube 51. A liquid contact ring 57 is attached to an end surface of the lining material. For parallel generation of the magnetic field in an entire range of a section of the measurement tube 51, the coils 52a and 52b are positioned and fixed in outer peripheral recesses 51a and 51b in the measurement tube through inner cores 58a and 58b, and are attached to the case 54 through outer cores 59a and 59b, and the brackets 55a and 55b.
The electrodes 53 are attached to the measurement tube 51 such that a distal end of an electrode rod 53a can be positioned inside the measurement tube 51. Further, a signal line (not shown) is attached to the electrode rod 53a.
The electromagnetic flowmeter 50 is interposed between flanges 60-1a and 60-2a at end surfaces of ducts 60-1 and 60-2, and is connected and fixed through coupling members including a screw rod 61 extending between the flanges 60-1a and 60-2a and a nut 62.
An electromagnetic flowmeter 150 shown in FIG. 5 is interposed between flanges 11-1a and 11-2a at end surfaces of ducts 11-1 and 11-2, and is connected and fixed through coupling members including a screw rod 61 extending between the flanges 11-1a and 11-2a and a nut 62. In FIG. 5, reference numeral 154 shows a terminal box.
A description will now be given of the operation with reference to FIG. 6.
The coil 52a is excited by power fed from an unillustrated power supply, thereby generating the magnetic field in the direction perpendicular to an axis of the measurement tube 51. When the measured fluid moves through the measurement tube 51 in the magnetic field, electromotive force is generated by Faraday's law of induction. In this case, the magnetic field is generated in the direction perpendicular to the electrically insulated measurement tube. Measurable voltage is generated between the pair of electrodes 53 unless the flowing liquid has excessively low conductivity. The voltage is proportional to the intensity of the magnetic field, a mean flow velocity of the fluid, and a distance between the electrodes. Thus, it is possible to measure a flow rate by converting the voltage into a signal according to the flow rate in a converter 63.
That is, according to the Faraday's law, the magnitude of induced voltage can be expressed by the following expression: EQU E=kBDv (1)
where
B: magnetic flux density (T) PA0 D: bore of measurement tube (m) PA0 v: mean axial fluid velocity (m/s) PA0 E: signal electromotive force (V) PA0 k: constant PA0 Q: volume flow rate (m.sup.3 /s)
In case of a cylindrical measurement tube, the volume flow rate can be expressed by the following expression: EQU Q=(.pi.D.sup.2 /4).multidot.v (2)
Depending upon the relationship, the expression (1) can be expressed as the following expression (3): EQU Q=(.pi.D/4 kB).multidot.E (3)
If the magnetic flux density B is kept constant, the flow rate in the tube can be found by measuring the signal electromotive force E.
The conventional electromagnetic flowmeter has the above structure. Hence, when the ratio of a diameter of the coil to a bore diameter (of 200 mm or more) of the measurement tube 51 is low, there is a high degree of flexibility in coil design such as the use of a saddle-like coil extending along a tube path. That is, the coil can freely be designed. Further, it is possible to easily form the magnetic flux feedback circuit including the inner cores, the coils, and the outer cores.
However, in case of a small bore diameter (ranging from 10 to 20 mm) of the measurement tube 51, a coil size can not be reduced according to the bore diameter. This is because the coil should inevitably become large with respect to the bore diameter to obtain required magnetic flux density since the electromotive force is proportional to the product of the number of turns and current flowing in the coil. If the coil current is increased to reduce a coil size, the electromagnetic flowmeter used to continuously measure the flow rate runs counter to demands for reduced power consumption, and causes a problem from a safety standpoint at a time of use in dangerous atmospheres.
A description will now be given of a case where the coil current is halved to meet the demands for the reduced power consumption. In order to ensure desired generating magnetic flux with a current value varied, the number of turns should be doubled. However, if the number of turns is simply doubled, the coil requires a doubled overall length. Therefore, a value of resistance of the coil itself is also doubled, resulting in a fourfold increase in heat value.
In this case, there is one technique to keep the value of resistance constant by doubling a wire size. In such a case, since a size of the coil itself is doubled or more, as shown in FIG. 4 a vertically extending portion of the case has a longitudinal form such that the vertically extending portion can be accommodated within a pitch of the screw rod 61 extending between the flanges. Thus, when the outer core forming a part of the magnetic flux circuit includes a combination of two semicircular discs, the overall case is increased in size, thereby running counter to demands for a smaller case. On the other hand, when an outer core including a two-body structure is assembled along a longitudinal coil, the case itself must have a two-body structure. Thus, for example, a joint welded in the vicinity of an electrode portion may corrode, and corrosive fluid may enter the electrode portion, resulting in inconveniences in view of durability.
Further, since the case 54 is made of magnetic material such as iron forming a part of the magnetic circuit, a corrosion resistant coating is required for the case 54, resulting in an increase of manufacturing manpower and a higher cost. Furthermore, when a magnetic tool such as spanner is put on the case 54, partial magnetic flux of the magnetic flux feedback circuit is disturbed by the spanner since the case 54 forms a part of the magnetic circuit. Consequently, the magnetic field in the direction perpendicular to the measured fluid is attracted by the spanner, thereby disturbing an optimal stream of the magnetic flux with respect to the measured fluid.